1. Field of the Invention
The invention relates to a method of accurately measuring spherical aberration in a projection optical system in a reduction projection step and repeat exposure system (stepper) through the use of Levenson type mask patterns having different sizes.
2. Description of the Related Art
One of conventional methods of adjusting spherical aberration in a projection system is disclosed in Japanese Unexamined Patent Publication No. 2-278811. This method makes it possible to compensate for vertical spherical aberration in a projection system. In the method, vertical spherical aberration .DELTA.10 corresponding to 100% of an aperture in the projection system is designed to be equal to or smaller than 2.5.times..lambda./NA.sup.2, and is further designed to be greater than -0.5.times..DELTA.70, but smaller than 1.5.times..DELTA.70, wherein .DELTA.70 indicates spherical aberration corresponding to 70% of the aperture.
Since an optical image focused by the projection system is observed as a resist image resulted from multi-reflection occurring in the resist having a certain thickness, an optical image focused in an optical system having no aberration cannot always present the best resist image. Hence, the suggested method makes it possible to deepen a focus of depth without reduction in resolution, by adjusting vertical spherical aberration in a projection system into a certain range.
Japanese Unexamined Patent Publication No. 10-148926 has suggested a method of projecting a half-tone phase shift mask pattern in a reduced scale to thereby focus both a focal point on an optical axis in a projection system and a focal point not on an optical axis on a plane of a photosensitive plate. The half-tone phase shift mask has a transparent portion and a semi-transparent portion. The transparent portion has a thickness varying dependent on a height of an image from an optical axis.
In the method, there is prepared a half-tone phase shift mask having a certain degree of shift. A photosensitive late is exposed to light and developed through the use of the mask. Then, a relation between an image's height and a focal point is measured based on the result of development, and spherical aberration is measured based on the thus measured relation. Then, there is estimated a degree of phase shift, that is, a thickness of the transparent portion which would focus the focal point on a plane of the photosensitive plate. In the suggested method, a degree of phase shift and a focal point are plotted in a graph, using spherical aberration as a parameter, to thereby accomplish an optimal halftone phase shift mask.
Japanese Unexamined Patent Publication No. 10-176974 has suggested a method of measuring aberration in a projection system, comprising the steps of positioning a mask on an optical path, the mask including either a line and space pattern or an isolating line pattern, positioning a substrate on which a resist is coated, on a location at which a light is projected by the projection system, projecting the pattern onto the resist by means of the projection system to thereby expose the resist to light, developing the resist, and measuring a positional divergence of an image of either the line and space pattern or the isolating line pattern, the positional divergence being caused due to the resist formed on the substrate.
Japanese Unexamined Patent Publication No. 10-232185 has suggested a method of measuring aberration in a projection system, comprising the steps of positioning a mask on an optical path in the projection system, the mask including a line and space pattern having lines of the same width, positioning a substrate on which a resist is coated, on a location at which a light is projected by the projection system, projecting the line and space pattern onto the resist by means of the projection system to thereby expose the resist to light, developing the resist, and measuring aberration, based on a difference in a line width between the opposite patterns in a pitch direction of an image of the line and space pattern, the difference being caused due to the resist formed on the substrate.
A mask used for fabrication of LSI is generally designed to include patterns having different line widths and different space widths. That is, a mask generally includes both a line and space having a relatively wide line width and a relatively great space width, as illustrated in FIG. 1A, and a line and space having a relatively narrow line width and a relatively small space width, as illustrated in FIG. 1B.
FIG. 2 is a graph illustrating a best focal point of each of the line and space illustrated in FIG. 1A and the line and space illustrated in FIG. 1B. A difference between those best focal points (hereinafter, referred to as "best focus difference") is caused due to spherical aberration in a projection system. Hence, it is usually done to narrow spherical aberration of a projection lens, using the best focus difference as a parameter.
For instance, in a projection system in which KrF excimer laser is used as a light source, spherical aberration is attempted to be made smaller, based on the best focus difference between a line and space having a line width and a space width which are almost equal to a wavelength of KrF excimer laser, 0.24 micrometers, and a line and space having a line width and a space width which are greater than a wavelength of KrF excimer laser, for instance 0.40 micrometers.
However, it is quite difficult to accurately measure a best focal point, and hence, it would be unavoidable to measure a best focal point with an error. The reason why an error is unavoidable is that abrasion of a line and space pattern has to be taken into consideration as well as a dimension of the pattern in order to accurately measure a best focal point of the pattern, but such abrasion is quite difficult to measure. An error of a best focal point, caused by such abrasion, is about 0.1 micrometer.
In addition, a best focal point associated with spherical aberration is not sufficiently great. Specifically, an exposure system being adjusted would have spherical aberration of about 0.1.lambda., wherein .lambda. indicates a wavelength of a light, whereas a best focus difference is equal to about 0.15 micrometers when an ordinary mask is used.
FIG. 3A illustrates a positional relation between a pupil plane 50 and diffracted lights in a line and space having a relatively wide line width and a relatively great space width, and FIG. 3B illustrates the same in a line and space having a relatively narrow line width and a relatively small space width. Spherical aberration is caused by a difference in an optical path which difference is generated in dependence on a distance between a diffracted light and a center of a pupil plane. Since first-order diffracted lights are spaced away from a center of a pupil plane by different distances, an image-forming characteristic of a pattern formed by first-order diffracted lights provides data about spherical aberration.
As illustrated in FIG. 3A, a zero-order deffracted light 40, first-order diffracted lights 41 and second-order diffracted lights 42 are within a pupil plane 50 in a line and space having a relatively wide line width and a relatively great space width, and as illustrated in FIG. 3B, a zero-order diffracted light 60 and first-order diffracted lights 61 are within a pupil plane 70 in a line and space having a relatively narrow line width and a relatively small space width.
Those zero-order diffracted lights 40 and 60, the second-order diffracted lights 42, and other higher-order diffracted lights cancel data about the first-order diffracted lights 41 and 61. As a result, it is considered that the best focus difference becomes small.